The present invention is directed to a process and apparatus for the regeneration of acids by pyrolysis of a spent acid liquor solution. More particularly, the invention is directed to a process and apparatus for the heating of spent acids in a primary heating zone of a furnace followed by heating in a secondary roasting zone.
Pickling acids, such as hydrochloric acid and nitric acid, are used in the treatment of iron and other metals. Typically, the spent acids are regenerated for reuse in various processes to minimize waste and problems associated with disposal.
Several acid regeneration processes spray the spent acid through a furnace to pyrolyze or roast the various components and to produce acid vapors. The vapors are passed through an absorption column counter-currently to an absorption liquid which is typically water. The regenerated acid is then withdrawn through the bottom of the column. The pyrolysis exhaust gases are withdrawn from the head space of the absorption column by an external exhaust fan before discharging through a chimney.
Pickling acids of hydrochloric acid used to treat iron or steel produce numerous metal chlorides in the spent acid liquor. Many metal chlorides can be thermally converted into metal oxides and hydrogen chloride. The metal oxides are separated from the gaseous hydrogen chloride. The hydrogen chloride is then recovered as hydrochloric acid for regenerating the pickling acids.
The spent acid solutions are aqueous solutions that are sprayed as fine droplets into a heated reactor. The droplets are heated to evaporate the water from spent acid. After the water is evaporated, the metal compounds begin to decompose to the metal oxide. The droplets descend through the reactor in a free fall to a lower section in the reactor. The residence times are generally very short so that the droplets and metal compounds must be calcined by pyrolysis into the metal oxide and the acid vapor in a few seconds. The acid must be split from the metal oxide before the acid vapors can be removed from the head space in the reactor and the oxides removed from the bottom end.
The short retention time in the reactor always produces a certain amount of residual acid bonded to the metal salt. The residual acid is difficult to remove completely and remains in the oxide as an undesirable contaminant. To minimize residual acid in the metal oxide, the reactor temperature must be set sufficiently high to ensure that the larger droplets and the droplets in the center of the spraying cone are substantially calcined. There is a time delay in the conversion process starting at the outer edges of the spraying cone and moving inwards. As a result, the oxide is overheated and quality of the oxide deteriorates with the specific surface area becoming smaller. In addition, there is always a risk of higher or lower levels of acid vapors being discharged with the oxide.
One example of this type of process is disclosed in Austrian Patent No. 395,312. A disadvantage of this type of process is that evaporation and decomposition takes place in a single step at high temperatures causing considerable acid decomposition. When nitric acid is treated, high levels of nitric oxides (NO, NO2) are produced. In addition, considerable cooling is needed to compensate for the high temperatures and, in some instances, the addition of oxidants is necessary. A similar process is also disclosed in U.S. Pat. No. 5,149,515. In this process, the acid being regenerated is evaporated in a counter-current flow using the exhaust from the reactor and then decomposed in the reactor. An other example of processes for regenerating acids is disclosed in U.S. Pat. No. 4,049,788.
Accordingly, a continuing need exists in the industry for an improved process for regenerating spent acid liquors.
The present invention is directed to a process and apparatus for regenerating spent acids, and particularly pickling acids. More particularly, the invention relates to a process and apparatus for the spray roasting of spent acid liquor where the reactor has a first heating zone serving as an evaporation zone and a second heating zone serving as a roasting zone.
Accordingly, a primary object of the invention is to provide a process and apparatus for the complete and uniform roasting of a spent acid liquor and recovering the acid vapors.
Another object of the invention is to provide a process and apparatus for heating a spent acid liquor in a first heating zone of a reactor to separate a substantial portion of the acid vapors and water from the metal salts and transferring the metal salts to a second heating zone or roasting chamber for converting the metal salts to metal oxides.
A further object of the invention is to provide a process for the spray roasting of spent pickling acids and regenerating acid without the formation of waste water.
Another object of the invention is to provide a process for regenerating acid by heating a spent acid in a first heating zone to vaporize the acid substantially without decomposing the acid.
A further object of the invention is to provide a process of regenerating acid by heating spent acid in a first heating zone of a reactor to separate acid vapors from metal salts substantially without decomposing the metal salts or forming metal oxides in the first heating zone.
Still another object of the invention is to provide a process and apparatus for heating a spent acid liquor at first temperature to evaporate the water from the liquor at a sufficiently low temperature to prevent decomposition of the acid and to prevent the formation of metal oxides followed by heating the metal salts to a second temperature to oxidize the metal salts.
The objects of the invention are basically attained by providing a process for regenerating spent acid liquor, comprising the steps of feeding a spent acid solution into a first heating zone of a reactor and heating the spent acid solution at a first temperature sufficient to evaporate a substantial portion of the water from the acid solution and to form particles of metal compounds and acid vapors, feeding the particles of metal compounds to a second heating zone of the reactor and heating at a second temperature to decompose the metal compounds, wherein the second temperature is higher than the first temperature, and recovering and regenerating an acid solution from the acid vapors.
The objects of the invention are further attained by providing a process of regenerating an acid solution from a spent acid solution containing a metal compound. The process comprises the steps of feeding the spent acid solution into an evaporating chamber of a reactor and heating the spent acid solution at a first temperature sufficient to evaporate a substantial portion of water from the spent acid solution and forming acid vapors and metal salts substantially without decomposing the acid and substantially without forming metal oxides, feeding the metal salt to a pyrolysis chamber of the reactor and heating the metal salts to decompose the metal salts and produce a metal oxide, and recovering the acid vapors from the reactor and regenerating the acid solution.
These and other advantages and salient features of the invention will become apparent from the annexed drawings and the detailed description of the invention.